1. Field of the Invention
The present invention relates to a method for controlling an optical power level, which sets up an offset value for a reference optical power, and controls laser power levels required for individual operation modes of an optical disc apparatus based on the offset value.
2. Description of the Related Art
Typically, an optical disc apparatus records data on an optical disc such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) using a laser diode, or reproduces the data recorded on the optical disc.
A power level of the laser diode has been determined when initially setting up a laser power, and has been constantly maintained and generated by an APC (Automatic Power Control) circuit contained in an RF (Radio Frequency) IC (Integrated Circuit).
FIG. 1 is a circuit diagram for implementing an APC function of the APC circuit. As shown in FIG. 1, an RF IC 10 receives a feedback signal FPDO from a photo detector (not shown) contained in a pickup unit 20 and an output signal of a DAC (Digital Analog Converter) 102, and outputs its resultant signal while maintaining a constant voltage level at an operational amplifier (OP-AMP) 103.
The DAC 102 receives digital data (hereinafter referred to as a DAC value) from a microcomputer 30, and converts the digital data into analog data or analog signal. In this case, the microcomputer 30 differently sets up the DAC values for individual operation modes, i.e., a data recording mode, a data playback mode, and a disc discrimination mode.
In case of setting up laser power levels according to individual operation modes, the microcomputer 30 sets up the laser power levels on the basis of a DAC offset value. The DAC offset value is an offset value transmitted from the microcomputer 30 to the DAC 102 at a time when an output voltage is about to come into existence from the RF IC 10. If a level shift circuit 101 is further used for the APC circuit shown in FIG. 1, an output voltage shifted by a prescribed voltage is generated as a resultant signal from the RF IC 10.
FIG. 2 is a graph illustrating a conventional method for calculating the DAC offset value. Referring to FIG. 2, the DAC offset value is the X-coordinate of a point at which a linear function graph meets the X-axis. The linear function graph is formed by predetermined DAC values DAC1, and DAC2 transmitted from the microcomputer 30 to the DAC 102, laser power levels P1, and P2 generated from the pickup unit 20 upon receiving the predetermined DAC values DAC1, and DAC2, or feedback voltage levels FPDO1 and FPDO2.
The DAC offset value can be calculated by a linear equation shown in the following Equation 1:
                                          rDAC            offset                    =                                                    DAC                ⁢                                                                  ⁢                1                ×                P1                            -                              DAC                ⁢                                                                  ⁢                2                ×                P                ⁢                                                                  ⁢                2                                                                    P                ⁢                                                                  ⁢                2                            -                              P                ⁢                                                                  ⁢                1                                                    ,                            [                  Equation          ⁢                                          ⁢          1                ]            
where “rDACoffset” is a DAC offset value.
In the meantime, FIG. 3 is a graph illustrating another conventional method for calculating a DAC offset value. Feedback voltage levels F1 and F2 of a laser diode are proportional to output power values P1 and P2 of a laser, such that a DAC offset value can be calculated by the following Equation 2:
                                          rDAC            offset                    =                                                    DAC                ⁢                                                                  ⁢                1                ×                F                ⁢                                                                  ⁢                1                            -                              DAC                ⁢                                                                  ⁢                2                ×                F                ⁢                                                                  ⁢                2                                                                    F                ⁢                                                                  ⁢                2                            -                              F                ⁢                                                                  ⁢                1                                                    ,                            [                  Equation          ⁢                                          ⁢          2                ]            
where “rDACoffset” is a DAC offset value.
However, in case of calculating the DAC offset value as above, laser power levels P1, and P2 and feedback voltage levels FPDO1 and FPDO2 vary with characteristics and status of the pickup unit 20 shown in FIG. 1, resulting in unavoidably increasing deviations of resultant DAC offset values. Furthermore, because DAC values for individual operation modes, for example, a data recording mode, a data playback mode, and a disc discrimination mode are determined based on the DAC offset value, DAC values for individual operation modes vary with the DAC offset value, and laser power levels for individual operation modes cannot be appropriately determined.